Genomic libraries are necessary tools to isolate and characterize known or unknown genes. In studying lung carcinogenesis, we attempt to investigate the role of oncogenes and genes which are involved in the regulation of differentiation and in the process of transformation. We decided to construct genomic libraries from a transformed human lung carcinoma cell line A1146 and from normal human bronchial epithelial (NHBE) cells. In addition, a genomic library from human normal fibroblasts was constructed too. To assess the completeness of the libraries, both the A1146 and the NHBE libraries were screened for the presence of alpha and beta human chorionic gonadotropin genes by plaque hybridization.